Can your credit score be so low that you cant even get a co-signer?

I'm currently trying to purchase my first car,but my credit score is low.My grandfather will co-sign for me.His credit score is high and he already as established car credit.But the salesman said "I cant even do a co-signer.I dont understand, like whats the point of doing a co-signer if they wont let me.Is my score that bad to were I cant even use a co-signer?